In the transmission apparatus used for wireless communication, a power amplifier (PA) that amplifies a RF signal to be transmitted consumes power most among the components of the transmission apparatus. Thus, in the development of the transmission apparatus, improving the power efficiency of the power amplifier (PA) is an important challenge. In recent communication standard, linear modulation is mainstream for spectral efficiency improvement. In the linear modulation, requirements concerning signal distortion are strict.
Thus, in the power amplifier (PA), to maintain linearity, average output power is set so that instantaneous maximum output (peak) power can be equal to or less than saturated output power. In other words, in the power amplifier (PA), as the ratio of the peak power of the signal to be amplified to average power (Peak-to-Average Ratio, hereinafter abbreviated to PAR) takes a larger value, to maintain the linearity, the average output power must be set lower than the saturated output power.
Generally, however, the power amplifier (PA) is characterized in that as the ratio of the average output power to the saturated output power is lower, the ratio (power efficiency) of supply power supplied to the power amplifier (PA) to output power extracted from the power amplifier (PA) is lower. The reduction of the power efficiency runs counter to energy saving.
The PAR of the RF signal has a unique value for each communication standard. In recently used high-speed wireless communication such as CDMA (Code Division Multiple Access), WLAN (Wireless Local Area Network), terrestrial digital broadcasting, or LTE (Long Term Evolution), the PAR takes a large value of several dB to tens of dB. Such a size of the PAR is a cause of great reduction of the power efficiency of the power amplifier (PA).
In the power amplifier (PA), to solve the problem of the reduction of the power efficiency caused by the low average output power, a polar modulation technology has been actively studied in recent years.
FIG. 1 illustrates the example of the power amplifier of an Envelope Tracking (ET) method that is a type of a polar modulation technology.
According to the ET method, transmission signal data is input to input terminal 401 of polar modulator 411, amplitude component signal 403 of the transmission signal data is output to output terminal 402 of polar modulator 411, and RF modulation signal 408 including the amplitude component and the phase component of the transmission signal data in the carrier wave is output to output terminal 407 of polar modulator 411. Polar modulator 411 has a function of individually setting the output timings of amplitude component signal 403 and RF modulation signal 408 to desired values.
Power supply modulator 404 outputs amplitude component signal 405 obtained by amplifying amplitude component signal 403, and modulates power supply terminal 409 of RF-PA (Radio Frequency Power Amplifier) 406 based on amplitude component signal 405. RF modulation signal 408 output to output terminal 407 of polar modulator 411 is input to RF-PA 406. RF modulation signal 410 that includes the amplitude component and the phase component of the transmission signal data in the carrier wave and that is amplified is output to output terminal 412 of RF-PA 406.
According to the ET method, the voltage of power supply terminal 409 of RF-PA 406 is controlled according to the amplitude of RF modulation signal 410. Particularly, when RF modulation signal 410 is low output power, the voltage of power supply terminal 409 of RF-PA 406 is accordingly reduced. Thus, wasteful power consumption can be suppressed by limiting the amount of supply power from power supply modulator 404 to RF-PA 406 to a necessary minimum during the low output power.
In recent communication standards, to achieve higher-speed wireless communication, as described in Non-patent Literature 1, a Carrier Aggregation (CA) technology collecting a plurality of fragmented bands to utilize has been used. In this CA technology, by bundling the plurality of bands to secure a broadband, a high transmission speed can be achieved.
In an inter-band Non-contiguous CA mode in which carrier frequencies are greatly different from each other (difference Δf between carrier frequencies is sufficiently larger than modulation bandwidth fBB of RF signal of each carrier), communication stability can be improved by simultaneously performing communication at a plurality of carrier frequencies whose propagation characteristics are different. By applying the CA technology, when a plurality of business operators intermittently allocates bands or when the plurality of business operators shares a band, corresponding communication can be performed.
In the wireless communication system using the CA technology, a transmission apparatus and a transmission method that transmit the RF signals of a plurality of bands are necessary. In such a transmission apparatus, similarly, improving of power efficiency is required.
FIG. 2 is a diagram illustrating the functional configuration of a transmission apparatus disclosed as a wireless communication machine in Patent Literature 1. The transmission apparatus illustrated in FIG. 2 has a function of transmitting the RF signals of a plurality of bands and a function of improving power efficiency by applying the polar modulation technology.
Specifically, in the transmission apparatus illustrated in FIG. 2, a modulation signal generated by modulation signal generator 61 is converted from the signal of an orthogonal coordinate system into the signal of a polar coordinate system at polar control unit 62, and separated into a PM signal having phase information and an AM signal having amplitude information. The separated PM signal is used for phase modulation for frequency generator 11 by PM control unit 63. Similarly, the AM signal is used for power supply modulation for PA 21 and PA 31 by power supply modulator 64. In other words, the polar modulation technology, for increasing or decreasing supply power from power supply modulator 64 to PA 21 and PA 31 according to the increase or decrease of the output power of PA 21 and PA 31, is applied. Thus, the reduction of the power efficiency, even in a high back-off state where average output power is low, can be suppressed.
The transmission apparatus illustrated in FIG. 2 includes path selection switches 14 and 41 for switching GSM (Global System for Mobile Communication) signal path 20 including PA 21 and UMTS (Universal Mobile Telecommunications System) signal path 30 including PA 31. PA 21 amplifies the RF signal of the carrier frequency fc1 of a wireless communication system (GSM), while PA 31 amplifies the RF signal of the carrier frequency fc2 of a wireless communication system (UMTS). When communication is performed at the wireless communication system of the carrier frequency fc1, path selection switches 14 and 41 are switched so that PA 21 can input or output a RF signal based on a control signal from controller 15. When communication is performed at the wireless communication system of the carrier frequency fc2, path selection switches 14 and 41 are switched so that PA 31 can input or output a RF signal based on a control signal from controller 15.
The transmission apparatus illustrated in FIG. 2, which is not compatible with the CA technology for simultaneously outputting two desired frequency components fc1 and fc2, has a multi-band operation function of operating for one frequency by temporally switching the frequency components fc1 and fc2.
As in the case of the transmission apparatus illustrated in FIG. 2, Patent Literatures 2˜5 disclose technologies for maintaining high power efficiency even when average output power is set low by preparing the number of PAs equal to that of used bands, by allocating each PA for each band, by installing band selection switches on the input sides or the output sides of the PAs, by switching the band selection switches so that the PA corresponding to a currently used band can input or output a RF signal, and by applying a polar modulation technology for controlling supply power from a power source to each PA.